Re: [PATCH v2] migration: check magic value for deciding the mapping of channels

["manish.mishra" <manish.mishra@nutanix.com>]

On 16/11/22 4:57 pm, Daniel P. Berrangé wrote:
> On Wed, Nov 16, 2022 at 04:49:18PM +0530, manish.mishra wrote:
>> On 16/11/22 12:20 am, Daniel P. Berrangé wrote:
>>> On Tue, Nov 15, 2022 at 06:11:30PM +0000, Daniel P. Berrangé wrote:
>>>> On Mon, Nov 07, 2022 at 04:51:59PM +0000, manish.mishra wrote:
>>>>> Current logic assumes that channel connections on the destination side are
>>>>> always established in the same order as the source and the first one will
>>>>> always be the main channel followed by the multifid or post-copy
>>>>> preemption channel. This may not be always true, as even if a channel has a
>>>>> connection established on the source side it can be in the pending state on
>>>>> the destination side and a newer connection can be established first.
>>>>> Basically causing out of order mapping of channels on the destination side.
>>>>> Currently, all channels except post-copy preempt send a magic number, this
>>>>> patch uses that magic number to decide the type of channel. This logic is
>>>>> applicable only for precopy(multifd) live migration, as mentioned, the
>>>>> post-copy preempt channel does not send any magic number. Also, tls live
>>>>> migrations already does tls handshake before creating other channels, so
>>>>> this issue is not possible with tls, hence this logic is avoided for tls
>>>>> live migrations. This patch uses MSG_PEEK to check the magic number of
>>>>> channels so that current data/control stream management remains
>>>>> un-effected.
>>>>>
>>>>> Suggested-by: Daniel P. Berrangé<berrange@redhat.com>
>>>>> Signed-off-by: manish.mishra<manish.mishra@nutanix.com>
>>>>>
>>>>> v2:
>>>>>     TLS does not support MSG_PEEK, so V1 was broken for tls live
>>>>>     migrations. For tls live migration, while initializing main channel
>>>>>     tls handshake is done before we can create other channels, so this
>>>>>     issue is not possible for tls live migrations. In V2 added a check
>>>>>     to avoid checking magic number for tls live migration and fallback
>>>>>     to older method to decide mapping of channels on destination side.
>>>>> ---
>>>>>    include/io/channel.h     | 25 +++++++++++++++++++++++
>>>>>    io/channel-socket.c      | 27 ++++++++++++++++++++++++
>>>>>    io/channel.c             | 39 +++++++++++++++++++++++++++++++++++
>>>>>    migration/migration.c    | 44 +++++++++++++++++++++++++++++-----------
>>>>>    migration/multifd.c      | 12 ++++-------
>>>>>    migration/multifd.h      |  2 +-
>>>>>    migration/postcopy-ram.c |  5 +----
>>>>>    migration/postcopy-ram.h |  2 +-
>>>>>    8 files changed, 130 insertions(+), 26 deletions(-)
>>>> This should be two commits, because the 'io' and 'migration'
>>>> code are two separate subsystems in QEMU.
>>>>
>>>>> diff --git a/include/io/channel.h b/include/io/channel.h
>>>>> index c680ee7480..74177aeeea 100644
>>>>> --- a/include/io/channel.h
>>>>> +++ b/include/io/channel.h
>>>>> @@ -115,6 +115,10 @@ struct QIOChannelClass {
>>>>>                            int **fds,
>>>>>                            size_t *nfds,
>>>>>                            Error **errp);
>>>>> +    ssize_t (*io_read_peek)(QIOChannel *ioc,
>>>>> +                            void *buf,
>>>>> +                            size_t nbytes,
>>>>> +                            Error **errp);
>>>> This API should be called "io_read_peekv" and use
>>>> "const struct iovec *iov", such that is matches the
>>>> design of 'io_readv'.
>>>>
>>>> There should also be a QIOChannelFeature flag
>>>> registered to indicate whether a given channel
>>>> impl supports peeking at data.
>>>>
>>>>
>>>>> @@ -475,6 +479,27 @@ int qio_channel_write_all(QIOChannel *ioc,
>>>>>                              size_t buflen,
>>>>>                              Error **errp);
>>>>> +/**
>>>>> + * qio_channel_read_peek_all:
>>>>> + * @ioc: the channel object
>>>>> + * @buf: the memory region to read in data
>>>>> + * @nbytes: the number of bytes to read
>>>>> + * @errp: pointer to a NULL-initialized error object
>>>>> + *
>>>>> + * Read given @nbytes data from peek of channel into
>>>>> + * memory region @buf.
>>>>> + *
>>>>> + * The function will be blocked until read size is
>>>>> + * equal to requested size.
>>>>> + *
>>>>> + * Returns: 1 if all bytes were read, 0 if end-of-file
>>>>> + *          occurs without data, or -1 on error
>>>>> + */
>>>>> +int qio_channel_read_peek_all(QIOChannel *ioc,
>>>>> +                              void* buf,
>>>>> +                              size_t nbytes,
>>>>> +                              Error **errp);
>>>> There should be qio_channel_read_peek, qio_channel_read_peekv,
>>>> qio_channel_read_peek_all and qio_channel_read_peekv_all.
>>> Actually ignore that.  We should not add any new APIs at
>>> all.  Instead the io_readv callback, and the qio_channel_read*all()
>>> methods should gain a 'int flags' parameter, in the same way that
>>> the write methods have one. Then there should be as
>>> QIO_CHANNEL_READ_FLAG_PEEK constant defined.
>> Hi Daniel, As MSG_PEEK always reads from top even if there were
>> previos partial reads, so current |qio_channel_readv_all_eofmay
>> not work? I can keep things upto ||qio_channel_readv as common
>> for both with flags parameters but have separate
>> ||qio_channel_readv_peek_all_eof? Does something like that works.|||
> Simplest is probably to just not add 'flags' to the 'all' variants,
> just the non-'all' varants.


sure Daniel, will do that. Thanks.

> With regards,
> Daniel